A major challenge in IR projector array technology is to produce a high-emittance structure that requires relatively little electrical power during operation. Resistor arrays are one popular approach to obtaining IR scene projectors capable of wide dynamic range.
Typically, an array designed for projecting radiation in the IR spectrum will have a large number of discrete pixel structures coupled to drive electronics. Arrays can be fabricated with a wide range of pixel sizes and pitches to meet the requirements of a specific optical systems. A representative array of the prior art may have 96 pixels and operates in the short-wavelength IR (SWIR) to the long-wavelength IR (LWIR) wavebands. The arrays may be optimized to a desired wavelength or projection purpose, such as generating dynamic radiation clutter scenes or multiple independently moving targets against a background that simulates conditions in the upper atmosphere of Earth. Variations in the thermal and electrical properties of the array are achievable via modification of the pixel, considering collimator optics and field of view (FOV) requirements of the desired application.
A critical parameter of thermal radiating projection elements is known as the thermal time constant "T", defined by "T"=C/G, where G is the thermal conductance between the heated element and a substrate associated with the element, and C is the thermal mass of the heated element. To achieve high-speed performance necessary to display dynamic IR scenes a pixel must have a short thermal time constant. Thus, a successful design will exhibit a thermal time constant that is a fraction of the frame rate at which the projector operates. Given as thermal time constant of millisecond duration, the pixels must be heated to many hundreds of degrees above ambient temperature to display high radiance scenes using only milliwatts of power per pixel. However, thermal isolation between adjacent pixels and the substrate associated with the pixel must be maintained to limit cross talk among adjacent pixels in the array.
The array electronics are relied upon to control pixel temperatures and maintain temperatures between frame updates for reduced image flicker. Traditionally located beside the IR arrays, recent advanced in CMOS addressing electronics and fabrication techniques have lead to a two-level IR array structure with the electronics disposed beneath a pixel emitter associated therewith, so that high fill factors may be achieved with pixels covering virtually the entire surface of the IR array.